FIG. 9 is a cross-sectional view of a GaAs solar cell as an example of a prior art light responsive semiconductor element. In FIG. 9, reference numeral 1 designates an n type GaAs substrate (hereinafter referred to as n type substrate). An n type AlGaAs buffer layer (hereinafter referred to as buffer type) 2 is disposed on the n type substrate 1 by a metal organic chemical vapor deposition method (MOCVD method) or a molecular beam epitaxy method (MBE method). An n type GaAs layer 3 (hereinafter referred to as n type layer) is disposed on the buffer layer 2. A p type GaAs layer (hereinafter referred to as p type layer) 4 is disposed on the n type layer 3. A p type AlGaAs layer (hereinafter referred to as window layer) 5 is disposed on the p type layer 4. Anti-reflection films (hereinafter referred to as AR film) 6a and 6b comprising Si.sub.3 N.sub.4 or Ta.sub.2 O.sub.5 produced by a chemical vapor deposition method (CVD method) or a sputtering method are disposed on the window layer 5. A p side electrode 7 is disposed on the p type layer 4 through a contact hole which is opened in the AR film 6 and the window layer 5 by etching. Electrode 7 is deposited by an evaporation method or a sputtering method. An n side electrode 8 is disposed on the rear surface of the n type substrate 1.
The operation will be described.
Light which is incident on a solar cell generates charge carriers in the window layer 5, the p type layer 4, and the n type layer 3. Among these charge carriers, only the charge carriers which diffuse and reach the pn junction between the p type layer 4 and the n type layer 3, contribute to the photocurrent. Generally, the light absorption coefficient of a crystal has a wavelength dependency in which the shorter the wavelength, the higher the light absorption coefficient. Accordingly, charge carriers are likely to be generated at the shallow region near the surface. The pn junction of the solar cell is located in the effective light absorption path where most of the charge carriers are generated. Furthermore, the buffer layer 2 prevents the generated change carriers from diffusing toward the n type substrate 1 due to the potential barrier presented. Buffer layer 2 also functions as a BSF (Back Surface Field) which reflects the charge carriers which have reached the buffer layer toward the pn junction surface. By this BSF effect, the recombination rate of charge carriers (electrons and holes) at the rear surface of the buffer layer 2 is reduced, thereby resulting in a reduction in reverse saturation current and an increase in open-circuit voltage. This buffer layer 2 is located at a position deeper than the depth of effective light absorption. The distance from this buffer layer 2 to the pn junction is established within a diffusion length of the charge carriers to enhance the effect of BSF.
In the prior art charge generation element so constructed, when the diffusion length of light carriers is reduced by a crystal defect produced by irradiation, even if the charge diffusing toward the n type substrate 1 are reflected the BSF effect of the buffer layer 2, they cannot reach the pn junction surface. In such a case, the BSF effect of the buffer layer 2 cannot be utilized effectively, thereby resulting in a reduction of the radiation resistance of the solar cell.
On the other hand, when the buffer layer 2 is in a shallow position to obtain sufficient BSF effect to enhance radiation resistance regardless of a little reduction in of the diffusion length, the effective light absorption depth is decreased, and therefore sufficient light absorption is not obtained, resulting in a reduced photocurrent. This also results in a reduced initial efficiency.